Transistors can be divided into two main types: bipolar junction transistors and field-effect transistors. Both types share a common structure comprising three electrodes with a semi-conductive material disposed therebetween in a channel region. The three electrodes of a bipolar junction transistor are known as the emitter, collector and base, whereas in a field-effect transistor the three electrodes are known as the source, drain and gate. Bipolar junction transistors may be described as current-operated devices as the current between the emitter and collector is controlled by the current flowing between the base and emitter. In contrast, field-effect transistors may be described as voltage-operated devices as the current flowing between source and drain is controlled by the voltage between the gate and the source.
Transistors can also be classified as p-type and n-type according to whether they comprise semi-conductive material which conducts positive charge carriers (holes) or negative charge carriers (electrons) respectively. The semi-conductive material may be selected according to its ability to accept, conduct, and donate charge. The ability of the semi-conductive material to accept, conduct, and donate holes or electrons can be enhanced by doping the material. The material used for the source and drain electrodes can also be selected according to its ability to accept and inject holes or electrodes. For example, a p-type transistor device can be formed by selecting a semi-conductive material which is efficient at accepting, conducting, and donating holes, and selecting a material for the source and drain electrodes which is efficient at injecting and accepting holes from the semi-conductive material. Good energy-level matching of the Fermi-level in the electrodes with the HOMO (Highest Occupied Molecular Orbital) level of the semi-conductive material can enhance hole injection and acceptance. In contrast, an n-type transistor device can be formed by selecting a semi-conductive material which is efficient at accepting, conducting, and donating electrons, and selecting a material for the source and drain electrodes which is efficient at injecting electrons into, and accepting electrons from, the semi-conductive material. Good energy-level matching of the Fermi-level in the electrodes with the LUMO (Lowest Unoccupied Molecular Orbital) level of the semi-conductive material can enhance electron injection and acceptance.
Transistors can be formed by depositing the components in thin films to form thin film transistors. When an organic material is used as the semi-conductive material in such a device, it is known as an organic thin film transistor.
Various arrangements for organic thin film transistors are known. One such device is an insulated gate field-effect transistor which comprises source and drain electrodes with a semi-conductive material disposed therebetween in a channel region, a gate electrode disposed adjacent the semi-conductive material and a layer of insulting material disposed between the gate electrode and the semi-conductive material in the channel region.
An example of such an organic thin film transistor is shown in FIG. 1. The illustrated structure may be deposited on a substrate (not shown) and comprises source and drain electrodes 2, 4 which are spaced apart with a channel region 6 located therebetween. An organic semiconductor (OSC) 8 is deposited in the channel region 6 and may extend over at least a portion of the source and drain electrodes 2, 4. An insulating layer 10 of dielectric material is deposited over the organic semi-conductor 8 and may extend over at least a portion of the source and drain electrodes 2, 4. Finally, a gate electrode 12 is deposited over the insulating layer 10. The gate electrode 12 is located over the channel region 6 and may extend over at least a portion of the source and drain electrodes 2, 4.
The structure described above is known as a top-gate organic thin film transistor as the gate is located on a top side of the device. Alternatively, it is also known to provide the gate on a bottom side of the device to form a so-called bottom-gate organic thin film transistor.
An example of such a bottom-gate organic thin film transistor is shown in FIG. 2. In order to show more clearly the relationship between the structures illustrated in FIGS. 1 and 2, like reference numerals have been used for corresponding parts. The bottom-gate structure illustrated in FIG. 2 comprises a gate electrode 12 deposited on a substrate 1 with an insulating layer 10 of dielectric material deposited thereover. Source and drain electrodes 2, 4 are deposited over the insulating layer 10 of dielectric material. The source and drain electrodes 2, 4 are spaced apart with a channel region 6 located therebetween over the gate electrode. An organic semiconductor (OSC) 8 is deposited in the channel region 6 and may extend over at least a portion of the source and drain electrodes 2, 4.
The conductivity of the channel can be altered by the application of a voltage at the gate. In this way the transistor can be switched on and off using an applied gate voltage. The drain current that is achievable for a given voltage is dependent on the mobility of the charge carriers in the organic semiconductor in the active region of the device (channel between the source and drain electrodes). Thus, in order to achieve high drain currents with low operational voltages, organic thin film transistors must have an organic semiconductor which has highly mobile charge carriers in the channel region.
The application of organic thin film transistors is currently limited by the relatively low mobility of organic semiconductor materials. It has been found that one of the most effective means of improving mobility is to encourage the organic material to order and align. The highest mobility organic semiconductor materials in thin film transistors show substantial ordering and crystallization, which is evident from optical micrography and X-ray diffraction.
Techniques for enhancing crystallization of the organic semiconductor in an organic thin film transistor include: (i) thermal annealing of the organic thin film transistor after deposition of the organic semiconductor; and (ii) designing the organic semiconductor molecules such that the organic semiconductor inherently has an increased ability to crystallize after deposition.
There are some problems with the aforementioned methods of enhancing crystallization in organic thin film transistor devices. One problem with the thermal annealing technique is that the device must be heated. This can damage components of the device, increase energy costs for the manufacturer, and increase the processing time required to manufacture such devices. One problem with the molecular design route is that it is time consuming and expensive to design new molecules with increased ability to crystallize. Furthermore, modifying the molecular structure of the organic semiconductor can detrimentally affect the functional properties of the material in the resulting thin film transistor. Additionally, modifying the molecular structure of the organic semiconductor can detrimentally affect the processability of the material during manufacture of organic thin film transistors. For example, the solubility of the material can be affected such that the material becomes difficult to solution process using deposition techniques such as spin coating or ink jet printing.
Some further problems are common to both the aforementioned techniques. One problem is that both techniques result in an increase in crystallization throughout the organic semiconductor layer. It may not be desirable to increase the crystallinity, and thus the conductivity, of the organic semiconductor in certain regions of an organic thin film transistor as this may lead to current leakage and shorting problems between underlying and overlying metallization. As such, it would be advantageous to provide a method of increasing crystallisation of the organic semiconductor only in desired regions of an organic thin film transistor.
Furthermore, neither technique allows the orientation of the organic crystals to be readily controllable as the semiconductor crystallizes. This is important as the conductivity of an organic semiconductor is sensitive to the orientation of the organic crystals. If the organic crystals are aligned then the conductivity of the semiconductor will vary according to the direction of current flow relative to the orientation of the organic crystals.
It should be noted at this point that the orientation of maximum conductance is not necessarily parallel to the orientation of the organic crystals. Nor is it necessarily perpendicular to the orientation of the organic crystals. In fact, the direction of maximum conductance will depend on the structure of the organic molecules within the crystals. However, this is testable for a particular molecule by passing current through the material in various different angular directions and measuring the conductance. For certain materials there may even be several different crystal orientations at which a maximum conductivity is achieved with local minimum conductivity at angular orientations therebetween.
While not being bound by theory, conduction through an organic semiconductor material will occur via two mechanisms: (1) conductions along the organic molecules; and (2) inter-molecular hopping between molecules. Thus, if the dominant mechanism for conduction is along the organic molecules then the direction of maximum conductance will tend to be fairly well aligned with the molecular orientation. Alternatively, if the dominant mechanism for conduction is hopping between molecules then the direction of maximum conductance will tend to correspond to the direction in which it is easiest for charge to hop from one molecule to the next. This will often be in a more perpendicular direction to the molecular orientation due to better—orbital overlap between adjacent molecules in a sideways direction.
The contribution of the two mechanisms will depend on the molecular structure of the organic semiconductor. For example, if very long chain semiconductive polymers are utilized then conduction along the organic molecules may dominate. Thus, if the polymers have a chain length equivalent to, or longer than, the distance between the source and drain electrodes, then the direction of maximum conductance will tend to coincide with the polymers being aligned in a direction parallel to a line connecting the source and drain electrodes. However, if much shorter molecules are utilized then inter-molecular hopping will begin to dominate. In this instance, there may be little orbital overlap between “end-on” molecules and conductivity in the aligned direction may be low. Accordingly, it may be beneficial to orientate the crystals in a direction perpendicular to a line connecting the source and drain electrodes. However, this may lead to a large number of intermolecular hops being required for charge to pass between the source and drain. Accordingly, for molecules much shorter than the distance between source and drain electrodes, a crystal orientation inclined at an angle somewhere between perpendicular and parallel to a line connecting the source and drain electrodes will correspond to the orientation of maximum conductance.
A number of prior art documents disclose techniques for controlling the orientation of organic semiconductor molecules in the channel region of an organic thin film transistor. These are discussed below.
EP 1 684 360 A1 discloses that it is advantageous to control the orientation of organic semiconductor molecules such that a molecular axis of main chains thereof is oriented to be inclined with respect to a direction from the source to the drain electrode. In fabricating an organic thin film transistor having the aforementioned configuration, conjugated organic semiconductor molecules are dissolved in a predetermined solvent and the solution is applied on a substrate in which grooves are formed parallel to a desired orientation direction.
EP 1 679 752 A1 describes an arrangement in which the organic semiconductive layer of an organic thin film transistor is formed using a mixture of a semiconductive material and a liquid crystal material which is utilized to control the orientation of the semiconductive material. The direction of orientation is controlled by rubbing the surface on which the aforementioned mixture is to be deposited in a predetermined direction such as direction from source to drain electrode.
US 2007/0126003 describes that a self-assembled monolayer film may be used to control the orientation of organic semiconductor disposed thereon in a channel region of an organic thin film transistor. It is described that the organic semiconductor molecules are regularly orientated in crystal grains. It is further described that the organic semiconductive molecules disposed on the self-assembled monolayer film in the channel region have larger grains and have a higher orientation order than those in portions outside the channel region where no self-assembled monolayer film is provided.
US 2007/0117298 A1 discloses a method of manufacturing an organic thin film transistor using chemical seeding. It is described that a lyophobic material can be deposited around a channel region to contain an inkjet printed organic semiconductor deposited thereafter within the channel region. It is further described that a lyophobic pattern in the channel region can be provided in order to control crystallisation and molecular orientation of the organic semiconductor film.
US 2007/0012914 A1 discloses an organic thin film transistor in which a layer promoting crystallization is provided in a channel region with the organic semiconductor deposited thereon. It is disclosed that the organic semiconductor layer contains at least porphyrin having an organic silane structure and the crystallization promoting layer comprises at least a polysiloxane compound. The crystallization promoting layer is described as being a uniform film thickness. It is stated that crystallization is promoted by virtue of the effect of combination of the siloxane structure and the organic silane structure. It is further disclosed that each of the layers may be deposited from solution. The organic semiconductor may be deposited in a precursor form and heated in order to form the crystallized organic semiconductive layer. It is stated that the step of heating will play an important role in allowing the crystallization promoting functionality.
US 2006/0289859 A1 discloses an organic thin film transistor in which crystallization of the organic semiconductor is improved by providing a gate insulating layer comprising a mixture of an insulating polymer and a surface treating agent. The insulating polymer and the surface treating agent are deposited from solution as a mixture. Octadecytrichlorosilane is given as an example of the surface treating agent and polyvinyl phenol is given as an example of an insulating polymer. A cross-linking agent is also included in the mixture such that when the solution is deposited and dried, the components cross link.
US 2006/0208266 A1 discloses an organic thin film transistor in which a buffer layer is provided in the channel region prior to deposition of the organic semiconductor. The buffer layer is described as functioning to determine the orientation of the overlying organic semiconductor. The buffer layer is described as being made of an organic polymer material having a liquid crystal core. Prior to depositing the buffer layer, the surface is prepared by rubbing such that the molecules of the buffer layer are orientated in a specific direction prior to deposition of the organic semiconductor thereover. It is described that the buffer layer may be formed by preparing a solution of precursor materials, depositing these materials from solution and then polymerizing the materials to form the buffer layer.
US 2006/01135326 A1 discloses an organic thin film transistor comprising a liquid crystalline organic semiconductor material. A liquid crystal alignment layer is provided prior to deposition of the organic semiconductor material. It is described that the liquid crystal alignment layer may be one of: a layer prepared by coating a polyimide based material and then subjecting it to a rubbing treatment; a layer comprising a curing resin having minute unevenness; or a layer comprising a curing resin having minute unevenness wherein the crystal alignment layer and the base material are integrated.
US 2005/0029514 A1 discloses an organic thin film transistor in which a plurality of grooves are used to align an overlying organic semiconductor in the channel region.
In light of the above disclosures, it is evident that a large amount of work has been performed by many different groups with the aim of improving the crystalline arrangement of organic semiconductor material in an organic thin film transistor. However, none of the techniques are wholly successful in eradicating the need for charge hopping between crystal domains of the organic semiconductor material within a channel region of an organic thin film transistor. This is because all of the techniques result in crystal growth being initiated at a plurality of different points within the channel region. As a result, a plurality of crystal domains grow within the channel region, none of which stretch over the entire distance from the source to the drain electrode. Accordingly, any charge flowing between the source and drain will inevitably hit a crystal domain boundary and will be required to hop to an adjacent crystal domain to continue on its journey. Such crystal domain boundaries increase resistance to flow of current within the organic semiconductor resulting in lower charge mobility and lower conductance.
It is an aim of embodiments of the present invention to address the aforementioned problems.